A static transfer switch uses power semiconductors to selectably transfer a load between a plurality of power sources. Because there are no mechanical moving parts, the transfer can be completed rapidly. Static transfer switches are often used where a reliable and independent second source of power is available and it is necessary to protect the load from even a few alternating current (AC) power frequency cycles of interruption time in a primary power source, and from any voltage surges or sags in the primary power source. An example of such critical loads is electrical equipment and systems that are essential to emergency reactor shutdown in nuclear power-generating facilities, defined as “Class 1E” by the Institute of Electrical and Electronics Engineers (IEEE). Such electrical equipment and systems must meet certain predetermined seismic, aging and mechanical vibration standards.
There has been a gradual adoption of digital Uninterruptible Power Supply (UPS) systems by industries that utilize secure power equipment. With this transformation has followed constant refining and development of electronic components, strategies and topologies for the UPS systems. Using this approach for UPS systems, leveraging modern technologies such as Digital Signal Processing (DSP) microprocessors, can lead to higher power-to-volume ratios, tighter control of power supply waveform and power quality, higher system efficiencies, and significant cost advantages. However, the North American nuclear industry, using UPS systems in Class 1E applications, has justifiably and understandably been hesitant to adopt such changes in design approach for critical-safety UPS systems until sufficient stability, maturity and quantifiable reliability has been attained. Despite this hesitation, increasing maturity of digital UPS systems coupled with older component obsolescence is causing North American nuclear engineering teams to re-think secure power solutions and embrace digital UPS designs.
Before adopting the newer digital UPS technology for Class 1E applications, there are key areas where significant in-depth review, evaluation and analysis are required. This is necessary to ensure that the foundation blocks of safety, reliability and ruggedness that are synonymous with, for example, North American nuclear installations, are maintained. Key areas such as software and firmware are of particular interest for UPS installations where the equipment typically comprises hard-wired componentry employing analog control techniques in power electronics. This results in a dichotomy for agencies and operators alike: how best to leverage the advantages of older, proven hardware with its legacy and proven track record, with new digital techniques where cost, efficiency and space provide a number of advantages.
A drawback of current Class 1E UPS systems is that experience is limited using DSP controlled systems. Of course, there are processes and tools that can be used in development and qualification testing that may mitigate much of the risk. However, when addressing the key area of static switch design and control, with a single point of common coupling on the AC power output, there is logically a point of potential common-cause failure.
Modern implementations of static switches typically consist of matched pairs of inversely mounted silicon-controlled rectifiers (SCRs) for each phase of the inverter and the bypass. The transfers between the inverter and static switch are dictated by load and operating conditions. Most conventional UPS systems use one of the following approaches:
(a) Fully analog systems that detect the lack of output voltage of the output through a series of comparators, such as the SE/SS line of ferro-resonant systems available from Ametek Solidstate Controls of Columbus, Ohio (hereafter “Ametek”);
(b) Fully digital systems that use analog/digital converters and software to determine when to transfer; and
(c) Hybrid systems that use comparators to generate pulses representative of voltage failures and software to determine the transfer sequence.
However, it is important to insure that in case of any failure of the UPS, be it digital or analog, hardware or software, the load is transferred safely and reliably to an alternate (“bypass”) power source.